The durability and service performance of an asphalt roadway can be affected by a number of variables, including the asphalt mix composition that is used, the construction techniques employed in building the roadway, the overall weight, axle weight and tire size of the vehicles using the roadway, the number and speed of the vehicles and the temperature and other environmental factors under which the roadway is used. Most of these various factors are beyond the control of the road designer. Furthermore, as traffic has increased on the nation's highways and as high-pressure radial tires have become more commonly used on heavy trucks, wear and even deterioration of the roadways has accelerated. Such wear and deterioration may be evidenced by rutting, stripping and/or fatigue cracking of the asphalt concrete roadway, which will reduce the service life of the roadway, as well as the comfort of the drivers and passengers in the vehicles using the roadway. In addition, rutting may also contribute to safety hazards that may arise from an accumulation of water in the rutting paths. Such accumulation may lead to hydroplaning, or in appropriate weather conditions, icing.
The ability to predict such undesirable performance characteristics as rutting, stripping and fatigue cracking from a sample of a particular asphalt paving composition can lead to the development of improved and longer-lasting paving compositions. Consequently, methods and apparatus for use in predicting such tendencies in various asphalt compositions have assumed increasing importance and emphasis. Applicants themselves have developed methods and a preferred apparatus for testing pavement samples in order to predict the performance characteristics of particular asphalt concrete compositions, and such methods and apparatus are described in their U.S. patent application Ser. No. 08/689,602, which is entitled "Testing Machine For Pavement Samples" now U.S. Pat. No. 5,659,140. However, the validity of all of these methods and apparatus for testing asphalt pavement samples depends to a large extent on the nature of the samples which are tested and the degree to which the tests simulate actual conditions encountered by asphalt concrete pavement surfaces. Consequently, the degree to which the samples are representative of paving material in roadways, considering the methods and equipment employed in modern road construction, is very important.
Since the 1940s, most asphalt concrete mixtures used in paving in the United States have been designed according to either the Hveem or the Marshall mixture design criteria. Francis Hveem of the California Division of Highways developed the methodology that bears his name beginning in the late 1930s, and at about the same time, the Marshall mix design method was developed at the Mississippi Highway Department by Bruce Marshall.
During the late 1930s and 1940s, several new laboratory tests were developed by which samples of asphalt concrete mixtures prepared according to these methods could be tested to predict how such mix designs would perform under actual traffic and environmental conditions. Originally, these laboratory samples were prepared by a static loading or impact loading method. According to the static loading method, a quantity of asphalt concrete of a particular mix design was placed in a mold, and a static compressive force was applied to the mixture in the mold. In the impact loading method, a weighted hammer was used to apply repeated impact loads to the mixture in a mold. Usually, the mixture in the mold was compacted under repeated hammer blows applied first on one side and then on the other side of the mold. Different drop hammer weights or compressive loads were considered, as well as different numbers of blows per side, different hammer or compactor shapes and different mold shapes and constructions. Hveem continued to experiment in this area, as did the Triaxial Institute, a Project Committee of the American Society For Testing Materials (ASTM). Some of Hveem's later conclusions are reported in a report entitled "Application of the Triaxial Test to Bituminous Mixtures--Hveem Stabilometer Method", which was released bearing a date of Jul. 7, 1949. In addition, some of the findings of the Triaxial Institute were reported by its chairman, V. A. Endersby, in an undated paper entitled "Triaxial Testing and the Triaxial Institute".
These published findings of Hveem and the Triaxial Institute suggest that specimens of asphalt concrete prepared by a static loading or an impact loading method do not resemble actual road surfaces, because the structural alignment of the aggregate particles in a sample prepared by such methods is not comparable to the structural alignment of the aggregate particles in an actual roadway. It was theorized that rolling compaction and the rolling of vehicular tires under the load of a vehicle's weight impart a kneading action to the asphalt roadway which aligns the aggregate particles in the asphalt in a unique way, and that this unique alignment cannot be duplicated by static loading or impact loading methods. Consequently, Hveem developed a kneading compactor that employs a small tamper to sequentially compact adjacent areas of the upper surface of a sample of asphalt concrete mixture that is contained in a rotating mold.
Subsequently, a gyratory kneading compactor was developed, such as is described in U.S. Pat. No. 2,972,249 of McRae. According to this patent, an asphalt concrete mixture is placed within a mold and a compression plunger is utilized to apply a compression force to the upper surface of the mixture within the mold. As such force is applied, the mold is oscillated in a gyratory fashion without rotation. According to McRae, this apparatus applies the dual stresses of compression and distortion to the mixture, which together constitute kneading. This kneading, or distortion under compression, insures a frictional movement and realignment among the particles in the sample that is reportedly similar to that obtained under actual road conditions.
However, Applicants have determined that the known static loading, impact loading and gyratory compacting methods of sample preparation do not readily lend themselves to testing to predict the effects of environmental and traffic conditions on roadways constructed with a particular asphalt concrete mix designs according to modern road building techniques, especially those using vibratory compaction rollers.
It would be desirable, therefore, if a method and apparatus could be developed for preparing a test sample of an asphalt concrete mixture that would simulate the construction techniques employed in modern road building. It would also be desirable if such apparatus could be developed that would be simple and economical to operate.